The present invention relates to an improvement in a numerical control (hereinafter referred to as an NC) working system.
An NC working system serves to command and control the position of a tool with respect to an object to be worked (workpiece) by numerical information corresponding to the position, so as to perform machining of the workpiece. According to an NC working system, complex shapes can be formed easily and accurately with improved producibility. With respect to a working machine such as a lathe utilizing an NC working system, it has been proposed to use a lathe having double tool rests. FIG. 1 shows the outline of such a lathe.
In FIG. 1, a workpiece 12 is positioned and fixed by a chuck 10 rotatable about an axis of rotation (Z-axis), the workpiece 12 being supported at one end by a tip portion 14a of a tail stock 14. Cutting tools 20 and 22 for performing machining on the workpiece 12 are fixed on first and second capstan rests 16 and 18 (rests I, II) respectively. In machining the workpiece 12, the first and second capstan rests 16 and 18 are moved in the Z-direction as shown by an arrow, to cause the cutting tools 20 and 22 to machine the workpiece 12.
In FIG. 2, the period of working time of each of the first and second capstan rests 16 and 18 is shown in the form of a Gantt chart, the period of working time consisting of the periods of time of travelling and idling of each of the capstan rests. That is, the working period of the first capstan rest 16 consists of travelling periods 100-1-100-5 and idling periods 102-1 and 102-2, while the working period for the second capstan rest 18 consists of travelling periods 104-1-104-4 and idling periods 106-1-106-3.
In the conventional NC working system, there is a problem in that the working period is sometimes lengthy because the working period for each capstan rest is obtained by an automatic program, and actual working is performed according to the thus obtained period. For example, there is a disadvantage in some cases in that the working period is lengthy, because the travelling period 100-2 of the first capstan rest 16 is set after the travelling period 104-1 of the second capstan rest 18 as shown in FIG. 2, although both the travelling periods 104-1 and 100-2 can be set at the same time. Actual working is performed in accordance with the Gantt chart of FIG. 2, resulting in an increased working period.